This application is based upon and claims priority of Japanese Patent Application No. 2001-101442, filed on Mar. 30, 2001, the contents being incorporated herein by reference.
1. Field of the Invention
The present invention relates to a semiconductor device and a method for fabricating the same and, particularly, it is preferable to be applied to a high-frequency compound semiconductor device using a GaAs substrate or the like.
2. Description of the Related Art
Conventionally, when wires of MESFETs or the like formed on a semiconductor substrate are connected, as a method for connecting wires formed substantially on the same surface, particularly, a method of forming air bridge wiring is proposed.
An example of a semiconductor device having the air bridge structure is shown in FIG. 5.
Here, a circuit region in which a plurality of MESFETs (having a source 102, a drain 103, and an overhanging-shaped gate electrode 104) are provided substantially on the same surface of a semiconductor substrate 101 is formed, the sources 102 of adjacent MESFETs are connected with each other by air bridge wiring 105, and a hollow 107 is formed between the air bridge wiring 105 and constituents of the MESFET under the air bridge wiring 105.
The provision of such an air bridge structure reduces parasitic capacitance between the wires (the sources 102, in this case) so that characteristics of low noise and high gains can be obtained in a high-frequency region. Accordingly, the air bridge structure is preferable to be applied to a compound semiconductor device requiring a high-frequency characteristic.
In the semiconductor device having the air bridge structure shown in FIG. 5, the air bridge wiring 105 is exposed in order to give a high priority to the high-frequency characteristic.
However, in measuring various characteristics of fabricated semiconductor chips, when scratch is performed, for example, a metallic dust or the like easily occurs and it is assumed that the dust adheres to the air bridge wiring after a wafer process so as to cause troubles such as a poor connection or a break.
In order to suppress the occurrence of the above troubles and to improve reliability of the device, as shown in FIG. 6, it is required to form a thick air bridge protective film 106 over the entire surface including the air bridge wiring 105 after forming a series of various wires and the like by patterning as shown in FIG. 5.
In this case, however, there is a problem that a part between the wires is buried in the air bridge protective film in an area where a wiring pattern of the circuit region is dense although the part between the wires, in an area where the wiring pattern of the circuit region is sparse, is not buried even if the thick air bridge protective film is formed.
Especially, as shown in FIG. 5 and FIG. 6, in a case in which a structure in a relatively complicated shape, for example, an overhanging shape such as of a gate electrode 104, is formed in the circuit region, a film thickness of the air bridge protective film becomes insufficient in a part covering the structure, which results in forming the air bridge protective film in an ununiform film thickness as a whole.
In other words, since growth of a film thickness of the air bridge protective film varies according to a pattern of the structure in the circuit region, control of the film thickness becomes difficult, which sometimes makes it impossible to ensure accuracy of the air bridge protective film and to fabricate a device of high accuracy, causing a problem that obtaining an intended high-frequency characteristic is extremely difficult.
As described above, presently, although an air bridge structure is adopted to realize a high-frequency device, formation of a protective film to prevent troubles such as a poor connection and a break of air bridge wiring results in impairing a high-frequency characteristic.
The present invention is made in consideration of the above problem, and it is an object of the present invention to provide a semiconductor device and a method for fabricating the same, in which an air bridge structure is adopted while troubles such as a poor connection or a break of air bridge wiring can be surely prevented so as to realize a high-frequency characteristic as well as to sufficiently ensure high reliability of the device.
As a result of intensive studies, the inventor has thought of forms of the invention explained below.
A target semiconductor device of the present invention is a semiconductor device constituted in a manner in which a circuit region having a plurality of wires is formed on a semiconductor substrate, in which an air bridge electrically connecting predetermined wires among the wires in the circuit region is provided, an air bridge protective film covering the air bridge is formed, and at least a lower portion of the air bridge in the circuit region is a region over which the air bridge protective film is not formed.
Another constitution of the semiconductor device of the present invention comprises: a semiconductor substrate, a circuit region provided on the semiconductor substrate and having a plurality of wires; an air bridge electrically connecting predetermined wires among the wires in the circuit region; and an air bridge protective film covering only the air bridge.
A method for fabricating a semiconductor device according to the present invention comprises the steps of: forming a protective pattern in a shape covering a predetermined area on a semiconductor substrate and exposing an air bridge connecting portion at an end thereof; forming a conductive film covering the protective pattern; processing the conductive film to form air bridge wiring connected with the air bridge connecting portion following the shape of the protective pattern; forming an insulating film covering the conductive film; processing the insulating film to form an air bridge protective film having a predetermined shape covering the air bridge wiring; and removing the protective pattern to form a hollow between the air bridge wiring and the predetermined area.
Another constitution of the method for fabricating a semiconductor device according to the present invention comprises the steps of: forming a protective pattern in a shape covering a predetermined area on a semiconductor substrate and exposing an air bridge connecting portion at an end thereof; forming a conductive film covering the protective pattern; forming an insulating film covering the conductive film; processing the insulating film and the conductive film to form air bridge wiring connected with the air bridge connecting portion following the shape of the protective pattern and to form an air bridge protective film covering the air bridge wiring; and removing the protective pattern to form a hollow between the air bridge wiring and the predetermined area.